1. Field of the Invention
This invention relates to an improved optical information recording medium utilizable in the field of optical recording, and to an optical information recording method and a system for recording and/or reproducing information using the same.
2. Discussion of Background
An optical information recording medium such as an optical disk has an extremely large capacity, and information can be recorded in or reproduced from the recording medium without making physical contact with it. Furthermore, information recorded in the optical information recording medium is readily accessible. Owing to such advantages, the optical information recording medium has been widely utilized, for instance, for apparatus for filing documents, motion pictures and motionless pictures, and external storage and back-up memory devices for computers.
Conventional optical information recording media are typically classified into the following three types:
(a) an optical information recording medium comprising a recording layer of a thin film whose main component is a metal having a low melting point such as Te;
(b) an optical information recording medium comprising a recording layer consisting of a light-reflection layer and an organic light-absorption layer; and
(c) an optical information recording medium comprising a recording layer made of an organic pigment having high reflectance.
Since the melting point of the metal used in the recording layer of the above recording medium (a) is low, high preservability cannot be expected.
The recording layers of the recording medium (a) are further divided into two types, that is, a pitting type, and a phase-transition type. When information is recorded in a pitting-type recording layer, pits are formed on the surface of the recording layer by applying light thereto. In case of the phase-transition type, a change in the reflectance, which is caused by phase transition between crystal and amorphous of the metal, is applied to information recording.
The pitting-type recording layer has low resolving power, so that high recording density cannot be expected. Regarding the phase-transition-type recording layer, control of the conditions for production is quite complicated, and the price of the final product thus becomes extremely high.
A recording layer of the optical information recording medium (b) is prepared by coating an organic light-absorption layer on a metallic light-reflection layer. Information is recorded in the organic light-absorption layer, which is performed by forming pits on the layer by applying laser beams thereto.
The recording layer of this type consists of two layers, so that it cannot be easily prepared. In addition, recording or reproduction of information on the substrate side is not easy due to the presence of the light-reflection layer.
A recording layer of the optical information recording medium (c) is prepared by forming a organic-pigment thin layer having high reflectance on a substrate. The organic pigment has a high melting point or decomposition temperature and low thermal conductivity, so that a recording layer having high sensitivity, C/N ratio and reliability is obtainable. Furthermore, since the organic pigment layer can be readily formed on a substrate by means of coating, it can be efficiently mass-produced, and the price of the final product can thus be made lower.
However, the recording medium of this type cannot maintain high sensitivity and C/N ratio unless it is fabricated with an air-sandwich structure. It is therefore difficult to make the recording medium thin, and application of the recording medium to an optical flexible disk is thus quite hard. The recording media of (a) and (b) also suffer from the same problem.
With respect to an optical disk, a disk having a diameter of 30 cm was initially commercialized, and then the size has been made smaller to 20 cm, and even to 13 cm in diameter. Aiming at development of the market for personal-use, a target size of the optical disk is now 9 cm.
It is, however, extremely difficult to obtain a small and thin optical disk as far as it is fabricated with a conventional air-sandwich structure. In the case where a substrate having a thickness of 1.2 mm is employed, the thickness of the final disk becomes at least 3 mm, and it is not easy at all to prepare a disk thinner than 3 mm.
A conventional optical disk comprising a recording medium of the pitting type basically consists of a substrate, a recording layer and air. The air layer serves as a heat insulating layer, so that high thermal efficiency is ensured. Furthermore, since the air layer does not restrain vaporization, decomposition and scattering of the recording layer, which commonly take place when information is recorded, high sensitivity and C/N ratio are also ensured. Therefore, if a protective layer is formed instead of the air layer, the sensitivity and C/N ratio of the optical disk are highly deteriorated.